You are right Tom,
(SELECT ... FROM activity WHERE activity.productcode ~ '^m3') acts
yeilds 235 rows, with a total of 240 rows in activity table. I wonder why EXPLAIN estimates only 1
row? Are there any online docs on how the planner works?
BTW, I would like to buy a book that would help me tune up postgres performance. Which book would
you suggest?
Thanks,
Oleg
Tom Lane wrote:
> Oleg Lebedev <olebedev@waterford.org> writes:
> > Deleting from pg_statistic restored view performance to the way it was
> > before I ran vacuum analyze.
> > Below I attach two files that contain explain statistics for the view before
> > (progress_report.txt) and after (progress_report_analyzed.txt) I ran vacuum
> > analyze. The first one takes about 15 secs. to complete, the second one
> > takes 12-13 minutes to complete.
>
> Looks like the issue is the number of rows estimated to be obtained from
> the "activity" table:
>
> good plan:
>
> > -> Merge Join (cost=14644.00..14648.62 rows=4 width=128)
> > -> Sort (cost=11.02..11.02 rows=2 width=48)
> > -> Seq Scan on activity (cost=0.00..11.00 rows=2 width=48)
> > -> Sort (cost=14632.99..14632.99 rows=367 width=112)
> > -> Subquery Scan media_acts (cost=14553.17..14617.36 rows=367 width=112)
>
> bad plan:
>
> > -> Nested Loop (cost=14605.17..14686.99 rows=2 width=128)
> > -> Seq Scan on activity (cost=0.00..11.00 rows=1 width=48)
> > -> Subquery Scan media_acts (cost=14605.17..14671.27 rows=378 width=112)
>
> The plans for media_acts look about the same, so I have to guess that
> activity actually yields 50 or so rows, not just one. That doesn't
> hurt the mergejoin too much, but it is a killer for the nestloop.
>
> You showed the query as
> (SELECT ...
> FROM activity
> WHERE activity.productcode ~ '^m3') acts
> How many rows actually match activity.productcode ~ '^m3' ? How many
> rows altogether in activity?
>
> regards, tom lane